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Initial image calibration removes Cas.A, Cyg.A, Tau.A, and Vir.A, as well as The Sun, and most of the diffuse galactic plane (red dotted line) emission, which would otherwise dominate. A further calibration step 'bootstraps' from currently published low frequency catalogs to correct the flux scaling for each image.
The individual snap images AARTFAAC makes every second are used to detect bright low frequency transients as quickly as possible for followup observing in near real time. Mosaics such as this one allow us to monitor the total observable source population over timescales of months to years allowing us to monitor for long term variability. This also provides beautiful view of the night sky, with radio eyes!